Recently, organic photoreceptors have been widely used as electrophotographic photoreceptors. Organic photoreceptors have advantages compared to other photoreceptors such as that a material suited for various exposure light sources emitting various wavelengths light from visible rays to infrared rays can be easily developed, a material with pollution potential to the environment can be selected and the production cost thereof is low. However it also has drawbacks such as that the mechanical strength and chemical resistively are low and tending of degradation of the static property, and on the surface is easily damaged.
Electrical and mechanical stress is applied directly to the organic photoreceptor, hereinafter simply referred to as photoreceptor, by a charging means, a developing means and a cleaning means. Accordingly, resisting ability against the stress is required of the photoreceptor.
Furthermore, resistivity to counter abrasion and often damage as well as degradation caused by reactive oxygen such as ozone and nitrogen oxide generated during corona discharge is required of the photoreceptor.
A constitution of the photoreceptor is frequently employed to overcome such mechanical and chemical resistivity problems in which the organic photoreceptor is constituted by series of layers of a charge generation layer and a charge transportation layer, in which the charge transportation layer as the surface layer is made of a uniform layer having high strength and high resistivity against permeation of the reactive gas, and the thickness of the charge transportation layer is not less than 20 μm.
Another investigated approach to overcome the problems is a method which provides a protective layer having high strength on the surface of the photoreceptor, as disclosed in JP-A 6-118681. However, such methods to increase the thickness of the charge transportation layer or to provide a stronger protective layer causes problems of image sharpness since the carriers generated in the charge generation are diffused to the sides before arriving at the surface. In the field of digital copying machines, formation a high resolution image is required to meat the demand of high image quality. However, a suitable static latent image can not be obtained by such a layer structure, or in the presence the protective layers causing diffusion of the carriers.
It is necessary for high fidelity reproduction to sufficiently maintain the potential contrast between the exposed and the unexposed areas of the photoreceptor. It is also important to inhibit the diffusion of generated carriers before the arrival at the surface charge for reproducing the high fidelity image. When the ratio D/μ of the diffusion constant D to the drift mobility μ in the charge transportation layer is large, the effect of the diffusion on the degradation of the high density static latent image must be taken into argument. Consequently the degradation of the static latent image is increased when the thickness of the charge transportation layer is increased, as disclosed in “Nihon Gazou Gakkai Shi (Journal of Image Society of Japan)” Vol. 38. No. 4, p. 296.
A photoreceptor in which the thickness of the charge transportation layer is reduced to inhibit diffusion of the static latent image has been proposed, as disclosed in JP-A 5-119503. However, in practice an unclear image with lowered image density tends to form when an image is formed by employing an image forming apparatus in which such a proposed organic photoreceptor is used. Such a phenomenon is due to by lowering of the static potential accompanied with reduction of the static capacity of the photoreceptor. As a result, the developability and the image density are lowered. Particularly, problems occur in that the developability is lowered and the image density formed by the reversal development is insufficient, and a text image and photographic image having high sharpness can be obtained with difficulty only when a developer employing a small particle toner is used.
For recovering such lowering of the developability it is effective to increase the amount of charge per unit area of a thin layer organic photoreceptor so as to increase the electrical field intensity per unit layer thickness. However, both image defects of black spotting and white spotting tend to occur during reversal development when the electrical field intensity per unit thickness of the photoreceptor is raised. Namely, the image defect of black spotting tends to form since charge injection unrelated to the image is increased, in relation to the increase of the electrical field intensity in the photoreceptor. As methods to prevent such black spotting defects, countermeasures providing an interlayer and an electrical isolation layer on the surface of the conductive substrate by anodization to prevent the injection of free carriers. For example, a photoreceptor has been proposed which has a thin photoreceptor layer and an interlayer containing titanium oxide particles as disclosed in JP-A 2002-196522. However, black spotting cannot be sufficiently prevented during reversal development by the foregoing method which causes the increased electrical field intensity.
On the other hand, image sharpness tends to be degraded since an image defect, so called white spotting is formed when the electrical field intensity-is raised. It is assumed that the surface charge at the exposed areas does not disappear which is caused by filming of small toner particles. When the surface charge of the photoreceptor is increased, the ability to clean the toner is lowered due to a rise of Coulomb's force between the photoreceptor and the toner particles so that the white spotting tends to occur. The defect of white spotting easily occurs due to the increase in the surface area when extremely small particle toner is used.